Redefine a Go function

redefine_func_amd64.go

package main

import (
	"encoding/binary"
	"fmt"
	"reflect"
	"syscall"
	"time"
	"unsafe"
)

func myTimeNow() time.Time {
	return time.Date(2026, 1, 30, 17, 0, 0, 0, time.FixedZone("Somewhere", -5))
}

func main() {
	addr := reflect.ValueOf(time.Now).Pointer()
	buf := unsafe.Slice((*byte)(unsafe.Pointer(addr)), 5)

	mprotect(addr, len(buf), syscall.PROT_READ|syscall.PROT_WRITE|syscall.PROT_EXEC)

	buf[0] = 0xe9                                // JMP
	src := addr + 5                              // Where to jump from
	dest := reflect.ValueOf(myTimeNow).Pointer() // Where to jump to
	binary.LittleEndian.PutUint32(buf[1:], uint32(int32(dest-src)))
	
	mprotect(addr, len(buf), syscall.PROT_READ|syscall.PROT_EXEC)

	fmt.Println(time.Now().Format(time.Kitchen))
}

func mprotect(addr uintptr, length int, flags int) error {
	pageSize := syscall.Getpagesize()

	// Round address down to page boundary.
	pageStart := addr &^ (uintptr(syscall.Getpagesize()) - 1)

	// Round up to cover complete pages.
	regionSize := (int(addr-pageStart) + length + pageSize - 1) &^ (pageSize - 1)

	region := unsafe.Slice((*byte)(unsafe.Pointer(pageStart)), regionSize)
	return syscall.Mprotect(region, flags)
}

redefine_func_arm64.go

package main

import (
        "encoding/binary"
        "fmt"
        "reflect"
        "syscall"
        "time"
        "unsafe"
)

import "C"

func myTimeNow() time.Time {
        return time.Date(2026, 1, 30, 17, 0, 0, 0, time.FixedZone("Somewhere", -5))
}

func main() {
        addr := reflect.ValueOf(time.Now).Pointer()
        buf := unsafe.Slice((*byte)(unsafe.Pointer(addr)), 4)

        mprotect(addr, len(buf), syscall.PROT_READ|syscall.PROT_WRITE|syscall.PROT_EXEC)

        dest := reflect.ValueOf(myTimeNow).Pointer() // Where to jump to
        offset := int32(dest - addr)

        // Encode the instruction:
        // -----------------------------------
        // | 000101 | ... 26 bit address ... |
        // -----------------------------------
        inst := (5 << 26) | (uint32(offset>>2) & (1<<26 - 1))
        binary.LittleEndian.PutUint32(buf, inst)

        mprotect(addr, len(buf), syscall.PROT_READ|syscall.PROT_EXEC)
        cacheflush(buf)

        fmt.Println(time.Now().Format(time.Kitchen))
}

func mprotect(addr uintptr, length int, flags int) error {
	pageSize := syscall.Getpagesize()

	// Round address down to page boundary.
	pageStart := addr &^ (uintptr(syscall.Getpagesize()) - 1)

	// Round up to cover complete pages.
	regionSize := (int(addr-pageStart) + length + pageSize - 1) &^ (pageSize - 1)

	region := unsafe.Slice((*byte)(unsafe.Pointer(pageStart)), regionSize)
	return syscall.Mprotect(region, flags)
}

func cacheflush(buf []byte) {
        start := unsafe.Pointer(unsafe.SliceData(buf))
        end := unsafe.Pointer(uintptr(len(buf)) + uintptr(start))
        C.__builtin___clear_cache(start, end)
}